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</a></li><li class="dropdown-item"><!----> <a href="https://github.com/fangfeiyue" target="_blank" rel="noopener noreferrer" class="nav-link external">
  Github
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></div></div> <!----></nav>  <ul class="sidebar-links"><li><section class="sidebar-group depth-0"><p class="sidebar-heading open"><span>JS高级程序设计</span> <!----></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/books/JS高级程序设计/01.html" class="sidebar-link">简介</a></li><li><a href="/books/JS高级程序设计/02.html" class="sidebar-link">第二章、HTML中的JS</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/02.html#大纲" class="sidebar-link">大纲</a></li><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/02.html#思考" class="sidebar-link">思考</a></li></ul></li><li><a href="/books/JS高级程序设计/04.html" class="sidebar-link">思考</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/04.html#思考" class="sidebar-link">思考</a></li></ul></li><li><a href="/books/JS高级程序设计/05.html" class="active sidebar-link">第四章、变量、作用域与内存</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/05.html#原始值与引用值" class="sidebar-link">原始值与引用值</a></li><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/05.html#执行上下文与作用域" class="sidebar-link">执行上下文与作用域</a></li><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/05.html#垃圾回收" class="sidebar-link">垃圾回收</a></li><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/05.html#总结" class="sidebar-link">总结</a></li></ul></li><li><a href="/books/JS高级程序设计/08.html" class="sidebar-link">第七章、迭代器与生成器</a></li><li><a href="/books/JS高级程序设计/10.html" class="sidebar-link">第九章、代理和反射</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/10.html#代理基础" class="sidebar-link">代理基础</a></li><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/10.html#总结" class="sidebar-link">总结</a></li></ul></li><li><a href="/books/JS高级程序设计/12.html" class="sidebar-link">第十一章、期约与异步函数</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/12.html#异步编程" class="sidebar-link">异步编程</a></li><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/12.html#期约" class="sidebar-link">期约</a></li><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/12.html#异步函数" class="sidebar-link">异步函数</a></li><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/12.html#小结" class="sidebar-link">小结</a></li></ul></li><li><a href="/books/JS高级程序设计/13.html" class="sidebar-link">第十二章、BOM</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/13.html#window对象" class="sidebar-link">window对象</a></li><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/13.html#location对象" class="sidebar-link">location对象</a></li><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/13.html#navigator对象" class="sidebar-link">navigator对象</a></li><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/13.html#screen对象" class="sidebar-link">screen对象</a></li><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/13.html#history对象" class="sidebar-link">history对象</a></li></ul></li><li><a href="/books/JS高级程序设计/15.html" class="sidebar-link">第十四章、DOM</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/15.html#节点层级" class="sidebar-link">节点层级</a></li><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/15.html#dom编程" class="sidebar-link">DOM编程</a></li><li class="sidebar-sub-header"><a href="/books/JS高级程序设计/15.html#mutationobserver-接口" class="sidebar-link">MutationObserver 接口</a></li></ul></li><li><a href="/books/JS高级程序设计/24.html" class="sidebar-link">第二十三章、JSON</a></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><p>JavaScript变量是松散类型的，由于没有规则定义变量必须包含什么数据类型，变量的值和数据类型在脚本生命期内可以改变。</p> <h2 id="原始值与引用值"><a href="#原始值与引用值" class="header-anchor">#</a> 原始值与引用值</h2> <p>ECMAScript变量可以包含两种不同类型的数据：原始值和引用值。原始值（primitive value）就是最简单的数据，引用值（reference value）则是由多个值构成的对象。</p> <p>JS中原始值有6种：Undefined、Null、Boolean、Number、String和Symbol。保存原始值的变量是按值（by value）访问的。</p> <p>引用值是保存在内存中的对象，由于JS不允许直接访问内存位置，在操作对象时实际操作的是该对象的引用。</p> <h3 id="动态属性"><a href="#动态属性" class="header-anchor">#</a> 动态属性</h3> <p>对于引用值而言，可以随时添加、修改和删除其属性和方法</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">let</span> person <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
person<span class="token punctuation">.</span>name <span class="token operator">=</span> <span class="token string">&quot;Nicholas&quot;</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>person<span class="token punctuation">.</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// &quot;Nicholas&quot;</span>
</code></pre></div><p>原始值不动态增删属性</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">let</span> name <span class="token operator">=</span> <span class="token string">&quot;Nicholas&quot;</span><span class="token punctuation">;</span>
name<span class="token punctuation">.</span>age <span class="token operator">=</span> <span class="token number">27</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>name<span class="token punctuation">.</span>age<span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// undefined</span>
</code></pre></div><p>原始类型除了可以使用原始字面量形式进行初始化还可以使用new关键，如果使用的是new关键字，则JavaScript会创建一个Object类型的实例。</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">let</span> name1 <span class="token operator">=</span> <span class="token string">&quot;Nicholas&quot;</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> name2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">&quot;Matt&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
name1<span class="token punctuation">.</span>age <span class="token operator">=</span> <span class="token number">27</span><span class="token punctuation">;</span>
name2<span class="token punctuation">.</span>age <span class="token operator">=</span> <span class="token number">26</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>name1<span class="token punctuation">.</span>age<span class="token punctuation">)</span><span class="token punctuation">;</span>    <span class="token comment">// undefined</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>name2<span class="token punctuation">.</span>age<span class="token punctuation">)</span><span class="token punctuation">;</span>    <span class="token comment">// 26</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">typeof</span> name1<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// string</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">typeof</span> name2<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// object</span>
</code></pre></div><h3 id="复制值"><a href="#复制值" class="header-anchor">#</a> 复制值</h3> <h3 id="传递参数"><a href="#传递参数" class="header-anchor">#</a> 传递参数</h3> <p>ECMAScript中所有函数的参数都是按值传递的。</p> <h3 id="确定类型"><a href="#确定类型" class="header-anchor">#</a> 确定类型</h3> <p>typeof操作符最适合用来判断一个变量是否为原始类型，如果值是对象或null，那么typeof返回&quot;object&quot;。</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">let</span> s <span class="token operator">=</span> <span class="token string">&quot;Nicholas&quot;</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">22</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> u<span class="token punctuation">;</span>
<span class="token keyword">let</span> n <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> o <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">typeof</span> s<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// string</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">typeof</span> i<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// number</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">typeof</span> b<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// boolean</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">typeof</span> u<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">typeof</span> n<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// object</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">typeof</span> o<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// object</span>
</code></pre></div><p>引出有一个小问题，在JS中怎么判断一个值是不是数组</p> <ol><li>instanceof</li></ol> <div class="language-js extra-class"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token keyword">instanceof</span> <span class="token class-name">Array</span>
</code></pre></div><ol start="2"><li>Object.prototype.toString.call</li></ol> <div class="language-js extra-class"><pre class="language-js"><code><span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token string">'[object Array]'</span>
</code></pre></div><ol start="3"><li>constructor</li></ol> <div class="language-js extra-class"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span>constructor <span class="token operator">==</span> Array
</code></pre></div><p>由于constructor的属性是可以被改写的，所以使用这种方式判断并不一定安全。</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span>
arr<span class="token punctuation">.</span>constructor  <span class="token operator">=</span> Object
arr<span class="token punctuation">.</span>constructor <span class="token operator">==</span> Array <span class="token comment">// false</span>
arr<span class="token punctuation">.</span>constructor <span class="token operator">==</span> Object <span class="token comment">// true</span>
</code></pre></div><ol start="4"><li>Array.isArray</li></ol> <div class="language-js extra-class"><pre class="language-js"><code>Array<span class="token punctuation">.</span><span class="token function">isArray</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
</code></pre></div><p>使用Array.isArray判断一个值是否是数组更可靠，但是Array.isArray是ES5的方法老版本的浏览器可能存在兼容问题，这里我们可以简单封装一个方法</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> <span class="token function-variable function">isArray</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">arr</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>Array<span class="token punctuation">.</span>isArray<span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>arr<span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token string">'[object Array]'</span>
  <span class="token keyword">return</span> Array<span class="token punctuation">.</span><span class="token function">isArray</span><span class="token punctuation">(</span>arr<span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre></div><h2 id="执行上下文与作用域"><a href="#执行上下文与作用域" class="header-anchor">#</a> 执行上下文与作用域</h2> <p>变量或函数的上下文决定了它们可以访问哪些数据，以及它们的行为。每个上下文都有一个关联的变量对象（variable object），而这个上下文中定义的所有变量和函数都存在于这个对象上。</p> <p>在浏览器中，全局上下文就是我们常说的window对象，因此所有通过var定义的全局变量和函数都会成为window对象的属性和方法。使用let和const的顶级声明不会定义在全局上下文中，但在作用域链解析上效果是一样的。上下文在其所有代码都执行完毕后会被销毁，包括定义在它上面的所有变量和函数（全局上下文在应用程序退出前才会被销毁，比如关闭网页或退出浏览器）。</p> <h3 id="作用域链增强"><a href="#作用域链增强" class="header-anchor">#</a> 作用域链增强</h3> <p>某些语句会导致在作用域链前端临时添加一个上下文，这个上下文在代码执行后会被删除。</p> <ul><li>try/catch语句的catch块</li> <li>with语句
这两种情况下，都会在作用域链前端添加一个变量对象。对with语句来说，会向作用域链前端添加指定的对象；对catch语句而言，则会创建一个新的变量对象，这个变量对象会包含要抛出的错误对象的声明。</li></ul> <h3 id="变量声明"><a href="#变量声明" class="header-anchor">#</a> 变量声明</h3> <ol><li>使用var的函数作用域声明</li></ol> <p>在使用var声明变量时，变量会被自动添加到最接近的上下文。在函数中，最接近的上下文就是函数的局部上下文。在with语句中，最接近的上下文也是函数上下文。如果变量未经声明就被初始化了，那么它就会自动被添加到全局上下文。var声明会被拿到函数或全局作用域的顶部，位于作用域中所有代码之前。这个现象叫作“提升”。</p> <div class="language-js extra-class"><pre class="language-js"><code>console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
<span class="token keyword">var</span> name <span class="token operator">=</span> <span class="token string">'Jake'</span><span class="token punctuation">;</span>

<span class="token comment">// 等价于</span>
<span class="token keyword">var</span> name
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span>
name <span class="token operator">=</span> <span class="token string">'Jake'</span>


<span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
  <span class="token keyword">var</span> name <span class="token operator">=</span> <span class="token string">'Jake'</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// 等价于</span>

<span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">var</span> name
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
  name <span class="token operator">=</span> <span class="token string">'Jake'</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><ol start="2"><li>使用let的块级作用域声明</li></ol> <p>let声明的变量的作用域是块级的，块级作用域由最近的一对包含花括号{}界定。换句话说，if块、while块、function块，甚至连单独的块也是let声明变量的作用域。</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> a<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ReferenceError: a没有定义</span>

<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> b<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>b<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ReferenceError: b没有定义</span>

<span class="token keyword">function</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> c<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>c<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ReferenceError: c没有定义</span>
                <span class="token comment">// 这没什么可奇怪的</span>
                <span class="token comment">// var声明也会导致报错</span>

<span class="token comment">// 这不是对象字面量，而是一个独立的块</span>
<span class="token comment">// JavaScript解释器会根据其中内容识别出它来</span>
<span class="token punctuation">{</span>
  <span class="token keyword">let</span> d<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>d<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ReferenceError: d没有定义</span>
</code></pre></div><p>let与var的另一个不同之处是在同一作用域内不能声明两次。</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">var</span> a<span class="token punctuation">;</span>
<span class="token keyword">var</span> a<span class="token punctuation">;</span>
<span class="token comment">// 不会报错</span>

<span class="token punctuation">{</span>
  <span class="token keyword">let</span> b<span class="token punctuation">;</span>
  <span class="token keyword">let</span> b<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// SyntaxError: 标识符b已经声明过了</span>
</code></pre></div><p>let的行为非常适合在循环中声明迭代变量。使用var声明的迭代变量会泄漏到循环外部，这种情况应该避免。</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">var</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">10</span><span class="token punctuation">;</span> <span class="token operator">++</span>i<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 10</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> j <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> j <span class="token operator">&lt;</span> <span class="token number">10</span><span class="token punctuation">;</span> <span class="token operator">++</span>j<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>j<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ReferenceError: j没有定义</span>
</code></pre></div><p>let在JavaScript运行时中也会被提升，但由于“暂时性死区”（temporal dead zone）的缘故，实际上不能在声明之前使用let变量。</p> <ol start="3"><li>使用const的常量声明</li></ol> <p>使用const声明的变量必须同时初始化为某个值。一经声明，在其生命周期的任何时候都不能再重新赋予新值。</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> a<span class="token punctuation">;</span> <span class="token comment">// SyntaxError: 常量声明时没有初始化</span>

<span class="token keyword">const</span> b <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>b<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3</span>
b <span class="token operator">=</span> <span class="token number">4</span><span class="token punctuation">;</span> <span class="token comment">// TypeError: 给常量赋值</span>
</code></pre></div><p>赋值为对象的const变量不能再被重新赋值为其他引用值，但对象的键则不受限制。</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> o1 <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
o1 <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// TypeError: 给常量赋值</span>

<span class="token keyword">const</span> o2 <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
o2<span class="token punctuation">.</span>name <span class="token operator">=</span> <span class="token string">'Jake'</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>o2<span class="token punctuation">.</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 'Jake'</span>
</code></pre></div><p>如果想让整个对象都不能修改，可以使用Object.freeze()，这样再给属性赋值时虽然不会报错，但会静默失败：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> o3 <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">freeze</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
o3<span class="token punctuation">.</span>name <span class="token operator">=</span> <span class="token string">'Jake'</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>o3<span class="token punctuation">.</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// undefined</span>
</code></pre></div><h2 id="垃圾回收"><a href="#垃圾回收" class="header-anchor">#</a> 垃圾回收</h2> <p>如何标记未使用的变量也许有不同的实现方式。不过，在浏览器的发展史上，用到过两种主要的标记策略：标记清理和引用计数</p> <h3 id="标记清理"><a href="#标记清理" class="header-anchor">#</a> 标记清理</h3> <h3 id="引用计数"><a href="#引用计数" class="header-anchor">#</a> 引用计数</h3> <p>其思路是对每个值都记录它被引用的次数。声明变量并给它赋一个引用值时，这个值的引用数为1。如果同一个值又被赋给另一个变量，那么引用数加1。类似地，如果保存对该值引用的变量被其他值给覆盖了，那么引用数减1。当一个值的引用数为0时，就说明没办法再访问到这个值了，因此可以安全地收回其内存了。垃圾回收程序下次运行的时候就会释放引用数为0的值的内存。</p> <p>引用计数遇到的问题是：循环引用，如对象A有一个指针指向对象B，而对象B也引用了对象A。</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">problem</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> objectA <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">let</span> objectB <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

  objectA<span class="token punctuation">.</span>someOtherObject <span class="token operator">=</span> objectB<span class="token punctuation">;</span>
  objectB<span class="token punctuation">.</span>anotherObject <span class="token operator">=</span> objectA<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><h3 id="性能"><a href="#性能" class="header-anchor">#</a> 性能</h3> <p>垃圾回收程序会周期性运行，如果内存中分配了很多变量，则可能造成性能损失，因此垃圾回收的时间调度很重要。尤其是在内存有限的移动设备上，垃圾回收有可能会明显拖慢渲染的速度和帧速率。开发者不知道什么时候运行时会收集垃圾，因此最好的办法是在写代码时就要做到：无论什么时候开始收集垃圾，都能让它尽快结束工作。</p> <p>现代垃圾回收程序会基于对JavaScript运行时环境的探测来决定何时运行。探测机制因引擎而异，但基本上都是根据已分配对象的大小和数量来判断的。</p> <h2 id="总结"><a href="#总结" class="header-anchor">#</a> 总结</h2> <p>JavaScript变量可以保存两种类型的值：原始值和引用值。原始值可能是以下6种原始数据类型之一：Undefined、Null、Boolean、Number、String和Symbol。原始值和引用值有以下特点。</p> <ul><li>原始值大小固定，因此保存在栈内存上。</li> <li>从一个变量到另一个变量复制原始值会创建该值的第二个副本。</li> <li>引用值是对象，存储在堆内存上。</li> <li>包含引用值的变量实际上只包含指向相应对象的一个指针，而不是对象本身。</li> <li>从一个变量到另一个变量复制引用值只会复制指针，因此结果是两个变量都指向同一个对象。</li> <li>typeof操作符可以确定值的原始类型，而instanceof操作符用于确保值的引用类型。</li></ul> <p>任何变量（不管包含的是原始值还是引用值）都存在于某个执行上下文中（也称为作用域）。这个上下文（作用域）决定了变量的生命周期，以及它们可以访问代码的哪些部分。执行上下文可以总结如下。</p> <ul><li>执行上下文分全局上下文、函数上下文和块级上下文。</li> <li>代码执行流每进入一个新上下文，都会创建一个作用域链，用于搜索变量和函数。</li> <li>函数或块的局部上下文不仅可以访问自己作用域内的变量，而且也可以访问任何包含上下文乃至全局上下文中的变量。</li> <li>全局上下文只能访问全局上下文中的变量和函数，不能直接访问局部上下文中的任何数据。</li> <li>变量的执行上下文用于确定什么时候释放内存。</li></ul> <p>JavaScript是使用垃圾回收的编程语言，开发者不需要操心内存分配和回收。JavaScript的垃圾回收程序可以总结如下。</p> <ul><li>离开作用域的值会被自动标记为可回收，然后在垃圾回收期间被删除。</li> <li>主流的垃圾回收算法是标记清理，即先给当前不使用的值加上标记，再回来回收它们的内存。</li> <li>引用计数是另一种垃圾回收策略，需要记录值被引用了多少次。JavaScript引擎不再使用这种算法，但某些旧版本的- IE仍然会受这种算法的影响，原因是JavaScript会访问非原生JavaScript对象（如DOM元素）。</li> <li>引用计数在代码中存在循环引用时会出现问题。</li> <li>解除变量的引用不仅可以消除循环引用，而且对垃圾回收也有帮助。为促进内存回收，全局对象、全局对象的属性和循- 环引用都应该在不需要时解除引用。</li></ul></div> <footer class="page-edit"><!----> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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